Use system default location for cafile when using php fopen.

Use system default location for cafile when using php fopen.

Up to 5.5, PHP does not accept any certificates if cafile/capath
is not set. (From 5.6 it uses the system default CA budle, which is
going to be a better choice than anything we can guess.) So try
to guess the location of the system default CA bundle.

Won't work on windows, but that's a lost cause anyway because PHP
(pre-5.6) can't handle the windows CA file format.

Bug: T75203
Change-Id: I07736c150fe0783e09d297395ed25adf335edbd3